1. Field of the Invention
This invention pertains to a method for producing homogeneous advanced ceramics materials from abundant, low cost, natural clays by intercalation and heat treatment. These composite materials can be used for wear parts, heat exchangers, refractories and the like. Advanced ceramics play an important role in replacing critical and strategic minerals.
2. Description of the Prior Art
Carbides and nitrides have excellent properties such as high hardness, high thermal conductivity, good heat resistance, good thermal shock resistance, high mechanical strength, and good chemical stability. These properties have brought about their use as advanced ceramic materials. Most high strength advanced ceramics contain the elements of silicon, aluminum, oxygen, nitrogen, and/or carbon. One of these advanced ceramic materials is SiC and there are several known methods used to produce it including direct reaction of silicon with carbon; carbothermal reduction of silica with carbon; vapor phase reaction of hydrocarbon with silicon tetrachloride or silicon tetrahydrogenate; and thermal decomposition of organic silicon polymer. Similarly, there are several methods used to produce silicon/aluminum nitrides. These include direct nitridation of silicon/aluminum with nitrogen; a vapor phase reaction between silicon/aluminum halides and ammonia; a reaction between silica/alumina and ammonia; and carbothermal reduction and nitridation of silica/alumina with nitrogen.
Pure carbides and nitrides are very difficult to densify by sintering. Usually, the addition of a sintering aid is necessary to promote densification. These additions, however, will degrade the performance of the ceramic material at high temperature. The use of the carbide or nitride particles with another phase to make composites such as SiC+Al.sub.2 O.sub.3, and SiC+AlN and solid solutions including SiAlON and SiC.AlN are other ways to have dense sintered products and to improve ceramic performance at high temperature. It has now been found that these composites and solid solutions can be synthesized at low cost by carbothermal reduction and/or nitridation of clays in inert gases and/or a nitrogen atmosphere.
I. B. Cutler, et al in "Sinterable SiAlON Powder by Reaction of Clay with Carbon and Nitrogen" American Ceramic Society Bulletin, v. 58, No. 9 (1979) reported mixing kaolin, carbon black, and a catalyst of 1 percent iron together and fired the mixture at a temperature close to 1,450.degree. C. to synthesize SiAlON. This method has serious drawbacks since the iron catalyst will react with silicon to form iron silicides and physical mixing limits structural homogeneity which is critical to making substantially pure SiAlON. No matter how good the dispersion is, the homogeneity of a mixture resulting from physical mixing still is limited by the particle size. Iron silicides melt at about 1,200.degree. C. to 1,500.degree. C. and are detrimental to the high temperature performance of the SiAlON.
A. C. D. Chaklader et al, "Al.sub.2 O.sub.3 -SIC Composites from Aluminosilicate Precursors" in the J. of American Ceramic Society v. 75, No. 8 (1992) reported mixing aluminosilicate and carbon in stoichiometric proportions and firing the mixture above 1,550.degree. C. in Ar gas to obtain a SiC+Al.sub.2 O.sub.3 composite. The SiC morphology of the fired material was related to the morphology of the starting carbon source. For example, by adding graphite fiber, a SiC fiber would be synthesized. Spherical SiC particles would be synthesized from spherical graphite particles. Instead of physical mixing, C. Kato, et al, Communications of the American Ceramic Society, Vol. 67 No. 11, 1984, reported synthesizing Beta-SiAlON from montmorillonite that was intercalated with polyacrynitrile and reduced carbothermally. Montmorillonite was treated with 0.01 to 0.02N hexylammonium hydrochloride solution three times and immersed and intercalated with an acrylonitrile monomer for 24 hours. The complex was then heated at 50.degree. C. for 24 hours to polymerize the acrylonitrile. The intercalation compound was then further heat treated for cyclization of polyacrynitrile in air at 220.degree. C. for 48 hours, then fired to 1,150.degree. C. to get SiAlON along with beta-SiC and AlN. The drawbacks of this method are the long process time, acrylonitrile toxicity, and its expense.
U.S. Pat. No. 4,652,436 describes a method for making nitrides and carbides by intercalating a monomer or prepolymer into the interlamellar space of a natural mineral. The intercalated clay-monomer or prepolymer complex had several heat treatments at a temperature range from 80.degree. to 250.degree. C. for over 52 hours in order to polymerize the monomer and to acquire flame resistant properties. The intercalated clay polymer complex was then processed at a temperature ranging from 1,100.degree. to 1,700.degree. C. under a nitrogen or reducing atmosphere. The synthetic carbide and nitride materials disclosed in this patent include silicon carbides, titanium carbide, vanadium carbide, silicon nitride, aluminum nitride, molybdenum nitride, and SiAlON. This patent does not show the production of useful ceramic materials. The materials synthesized from clays were a mixture of SiC, Si.sub.3 N.sub.4, and SiAlON. Useful ceramic materials require suitable physical and chemical properties of each phase such as thermal expansion, thermal resistance, corrosion resistance, etc. Useful ceramic raw materials do not contain so many phases.
The composites of SiC and Al.sub.2 O.sub.3, and of SiC and AlN along with the solid solutions of SiAlON and of SiC.AlN are typical useful ceramic materials. They can be synthesized from the mixture of clays and carbon by heat treatment through carbothermal reaction and/or nitridation reaction. To produce useful raw materials for ceramic processing requires precise composition control of the carbon and clay mixture. In U.S. Pat. No. 4,652,436, carbon was derived from the decomposition of a polymer and the polymer was derived from the polymerization of a monomer by the use of an initiator or a catalyst at a low temperature range from 80.degree. to 250.degree. C. for over 52 hours.
Clays are also known to be catalysts (see A. C. D. Newman 1987 "Chemistry of Clays and Clay Minerals"). However, clays have variable compositions, impurities, and surface conditions. These factors cause the transformation of the polymer to carbon to be less reproducible or difficult to control (see D. H. Solomon and M. J. Rosser 1965 "Reactions Catalyzed by Minerals. Part I polymerization of Styrene" J. of Applied Polymer Science). In addition, a wide range of molecular weight can be produced during polymerization, causing difficulty in reproducing and controlling carbon concentration for later carbonization reaction and carbothermal reduction. Also, the polymerization process of U.S. Pat. No. 4,652,436 takes over 52 hours to complete and requires two filtration steps to remove chemicals. This increases the technical difficulties for his process, making it difficult to use commercially.
U.S. Pat. No. 4,652,436 uses polymers to derive carbon. Less controllable carbon concentration and long polymerization process time are drawbacks in the disclosure of U.S. Pat. No. 4,652,436. Polymerization is not a process step in this invention. Therefore, no process time is required for polymerization and carbon concentration is reproducible and easy to control. Also useful ceramic composites and/or solid solutions were synthesized. This invention represents an improvement because 1) it has increased the flexibility to include a wide variety of intercalated chemicals irrespective of their volatility; 2) intercalation for synthesis of advanced ceramics can be done in an aqueous media; 3) the process time to produce advanced materials is short compared to U.S. Pat. No. 4,652,436; 4) dehydrated clays can be intercalated and synthesized to produce advanced ceramic composites and solid solutions; 5) the concentration of carbon is reproducible and easy controllable.
The present invention therefore provides an improved process for the synthesis of advanced ceramics, such as SiC+AlN, SiAlON, SiC+Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4 +AlN from natural clays such as kaolin, halloysite, and montmorillonite by an intercalation and heat treatment method. Advanced ceramics produced by this process are low cost and highly homogeneous, i.e. nanostructured materials made using a simple manufacturing process. The product can be easily ground and requires a short synthesis time. The advanced ceramics produced by the process of this invention can be used in refractory ceramic and/or structured ceramic applications such as wear parts.